CSE 390a Lecture 5

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CSE 390aLecture 5

• Intro to shell scripting
• slides created by Marty Stepp, modified by
  Jessica Miller Ruth Anderson
• http//www.cs.washington.edu/390a/

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Lecture summary

• basic script syntax and running scripts
• shell variables and types
• control statements the for loop

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Shell scripts

• script A short program meant to perform a
  targeted task.
• a series of commands combined into one executable
  file
• shell script A script that is executed by a
  command-line shell.
• bash (like most shells) has syntax for writing
  script programs
• if your script becomes gt 100-150 lines, switch
  to a real language
• To write a bash script (in brief)
• type one or more commands into a file save it
• type a special header in the file to identify it
  as a script (next slide)
• enable execute permission on the file
• run it!

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Basic script syntax

• !interpreter
• written as the first line of an executable
  script causes a file to be treated as a script
  to be run by the given interpreter
• (we will use /bin/bash as our interpreter)
• Example A script that removes some files and
  then lists all files
• !/bin/bash
• rm output.txt
• ls -l

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Running a shell script

• by making it executable (most common
  recommended)
• chmod ux myscript.sh
• ./myscript.sh
• by launching a new shell
• bash myscript.sh
• by running it within the current shell
• source myscript.sh
• advantage any variables defined by the script
  remain in this shell(seen later)

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echo

• Example A script that prints your home
  directory.
• !/bin/bash
• echo "This is my amazing script!"
• echo "Your home dir is pwd"
• Exercise Write a script that when run on attu
  does the following
• clears the screen
• displays the current date/time
• Shows who is currently logged on info about
  processor

command description
echo produces its parameter(s) as output(the println of shell scripting) -n flag to remove newline (print vs println)
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Script example

• !/bin/bash
• clear please do not use clear in your hw
  scripts
• echo "Today's date is date"
• echo
• echo "These users are currently connected"
• w -h sort
• echo
• echo "This is uname -s on a uname -m
  processor."
• echo
• echo "This is the uptime information"
• uptime
• echo
• echo "That's all folks!"

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Comments

• comment text
• bash has only single-line comments there is no
  / ... / equivalent
• Example
• !/bin/bash
• Leonard's first script ever
• by Leonard Linux
• echo "This is my amazing script!"
• echo "The time is date"
• This is the part where I print my home
  directory
• echo "Home dir is pwd"

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Shell variables

• namevalue (declaration)
• must be written EXACTLY as shown no spaces
  allowed
• often given all-uppercase names by convention
• once set, the variable is in scope until unset
  (within the current shell)
• AGE64
• NAMEMichael Young"
• name (usage)
• echo "NAME is AGE years old"
• Michael Young is 64 years old

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Common errors

• if you misspell a variable's name, a new variable
  is created
• NAMERuth
• ...
• NameRob oops meant to change NAME
• if you use an undeclared variable, an empty value
  is used
• echo "Welcome, name" Welcome,
• when storing a multi-word string, must use quotes
• NAMERuth Anderson Wont work
• NAMERuth Anderson" NAME is Ruth Anderson

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More Errors

• Using during assignment or reassignment
• mystringHi there error
• mystring2Hello
• mystring2Goodbye error
• Forgetting echo to display a variable
• name
• echo name

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Capture command output

• variablecommand
• captures the output of command into the given
  variable
• Simple Example
• FILEls .txt
• echo FILE
• More Complex Example
• FILEls -1 .txt sort tail -1
• echo "Your last text file is FILE"
• What if we leave off the last backtick?
• What if we use quotes instead?

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Types and integers

• most variables are stored as strings
• operations on variables are done as string
  operations, not numeric
• to instead perform integer operations
• x42
• y15
• let z"x y" 57
• integer operators - /
• bc command can do more complex expressions
• if a non-numeric variable is used in numeric
  context, you'll get 0

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Bash vs. Java

• x3
• x vs. x vs. "x" vs. 'x' vs. \'x\' vs.
  'x'

Java Bash
String s "hello" shello
System.out.println("s") echo s
System.out.println(s) echo s
s s "s" // "hellos" sss
String s2 "25" String s3 "42" String s4 s2 s3 // "2542" int n Integer.parseInt(s2) Integer.parseInt(s3) // 67 s225 s342 s4s2s3 let n"s2 s3"
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Special variables

• these are automatically defined for you in every
  bash session
• Exercise Change your attu prompt to look like
  this
• jimmy_at_mylaptop
• See man bash for more details on setting your
  prompt

variable description
DISPLAY where to display graphical X-windows output
HOSTNAME name of computer you are using
HOME your home directory
PATH list of directories holding commands to execute
PS1 the shell's command prompt string
PWD your current directory
SHELL full path to your shell program
USER your user name
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PATH

• When you run a command, the shell looks for that
  program in all the directories defined in PATH
• Useful to add commonly used programs to the PATH
• Exercise modify the PATH so that we can
  directly run our shell script from anywhere
• echo PATH
• PATHPATH/homes/iws/rea
• What happens if we clear the PATH variable?

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set, unset, and export

• typing set or export with no parameters lists all
  variables
• Exercise set a local variable, and launch a new
  bash shell
• Can the new shell see the variable?
• Now go back and export. Result?

shell command description
set sets the value of a variable(not usually needed can just use x3 syntax)
unset deletes a variable and its value
export sets a variable and makes it visible to any programs launched by this shell
readonly sets a variable to be read-only(so that programs launched by this shell cannot change its value)
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Console I/O

• variables read from console are stored as strings
• Example
• !/bin/bash
• read -p "What is your name? " name
• read -p "How old are you? " age
• printf "10s is 4s years old" name age

shell command description
read reads value from console and stores it into a variable
echo prints output to console
printf prints complex formatted output to console
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Command-line arguments

• Example.sh
• !/bin/bash
• echo Name of script is 0
• echo Command line argument 1 is 1
• echo there are command line arguments _at_
• Example.sh argument1 argument2 argument3

variable description
0 name of this script
1, 2, 3, ... command-line arguments
number of arguments
_at_ array of all arguments
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for loops

• for name in value1 value2 ... valueN do
• commands
• done
• Note the semi-colon after the values!
• the pattern after in can be
• a hard-coded set of values you write in the
  script
• a set of file names produced as output from some
  command
• command line arguments _at_
• Exercise create a script that loops over every
  .txt file in the directory, renaming the file to
  .txt2
• for file in .txt do
• mv file file2
• done

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Exercise

• Write a script createhw.sh that creates
  directories named hw1, hw2, ... up to a maximum
  passed as a command-line argument.
• ./createhw.sh 8
• Copy criteria.txt into each assignment i as
  criteria(2i).txt
• Copy script.sh into each, and run it.
• output Script running on hw3 with criteria6.txt
  ...
• The followingcommand may behelpful

command description
seq outputs a sequence of numbers
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Exercise solution

• !/bin/bash
• Creates directories for a given number of
  assignments.
• for num in seq 1 do
• let CRITNUM"2 num"
• mkdir "hwnum"
• cp script.sh "hwnum/"
• cp criteria.txt "hwnum/criteriaCRITNUM.t
  xt"
• echo "Created hwnum."
• cd "hwnum/"
• bash ./script.sh
• cd ..
• done